As a result of the new demands of the market, components of internal combustion engine undergo greater demands and so they need to exhibit solutions capable of guaranteeing better performance, as well as to contribute to greater reliability and performance of the engine.
Additionally, the whole production chain of the world automobile industry has been challenged by the need to reduce atmospheric emissions generated by the burning of fossil fuels.
Thus, a number of makers of automotive components seek different technical solutions, chiefly for cylinders of internal combustion engines, among others. It should be noted that cylinders of internal combustion engines comprise both the cylinder liner applied to an engine block and the cylinders that are formed integrally with the engine block itself. For better understanding, only the term “cylinder” will be used from now on to define any of the possibilities mentioned.
Regardless of the technical solution, cylinders of internal combustion engine are engine components that undergo significant wear due to the type of work which they perform. This characteristic, coupled to the growing challenges for the automobile industry, result in the need to work with lower dimension tolerances, greater work pressures, more corrosive conditions, which result in effects on an engine component that have to be solved. It should be further noted that, in the case of engines that operate with Diesel cycle, this type of wear is quite high, particularly due to the presence of the sulfur element in the Diesel fuel.
Thus, the possible solutions that enable one to improve the performance of engines subjected to said conditions can be achieved through the improvement of the material used for producing cylinders, always taking into consideration the cost of such a solution. In this regard, there are some advances, markedly in the case of cylinders comprising ferrous alloys.
The main alloys applied in producing cylinder liners of the prior art are ferrous alloys. Among them, one can cite gray cast iron, which exhibits a reduced cost and, mainly, excellent tribological characteristics due to the presence of a large amount of solid lubricant, in the form of graphite, on the slide surface. Anyway, this material does not enable additional reduction of loses by friction or reduction of wear, which the present-day conditions require.
Document EP 1783349 discloses a solution for various internal components of engines, among which are cylinders, which enables a low friction coefficient. For this purpose, the internal peripheral surface of a metallic cylinder body receives a hard carbon film by chemical vapor deposition (CVD), the hard carbon film being provided with the silicon element in an amount ranging from 1% to 20%, that is, the cylinder work surface comprises an amount of silicon that may range from 1% to 20%. Moreover, said film has thickness ranging from 2 μm and 5 μm, and the roughness should be lower than Rz 0.5 μm.
Even though the proposed solution exhibits characteristics that result in a low friction coefficient and good wear resistance, there is a great drawback. The low roughness achieved (lower than 0.5 Rz) imparts to the cylinder work surface such a fine finish that it prevents good lubrication, for the simple reason that there is no sufficient roughness—mainly little presence of valleys—for accumulating oil, giving rise to an almost dry work surface and premature wear of the film. This situation is aggravated by the fact that the hard carbon film is not porous, which impairs the lubrication, increasing the premature wear of the film. The application of this technical solution will be even more disadvantageous on an engine that operates on Diesel cycle, since the work pressures are significantly higher.
Document EP1510594 also discloses a cylinder of an internal combustion engine, the internal peripheral surface of which receives a hard carbon film. Said film has thickness ranging from 0.3 μm and 2 μm and roughness lower than 0.1 μm for the parameter Ra, its hardness ranging from 1000 Hv to 3500 Hv. According to this document, roughness higher than Ra 0.1 μm may result in increase in the friction coefficient. As mentioned in the previous document, this solution presents, as a disadvantage, deficient lubrication due to the reduced roughness of the film, incapable of accumulating lubricating fluid properly, which leads to premature wear of the film, chiefly on internal combustion engines that operate under high pressures (higher than 6 MPa (60 bar).
In addition to the problems resulting from the prior-art technologies mentioned, it should be noted that excessive roughness of hard carbon film (higher than Rz 4.0 μm) also entails drawbacks. A high roughness generates cracks followed by delamination due to the high contact pressure between the roughness on the slide surfaces of the rings and pistons, which leads to the premature failure of the film.
FIG. 1 shows results of tests of engines with cylinders made of cast iron, provided with a hard carbon film on their internal peripheral surface. The surface roughness, FIGS. 1A and 1B, after coating with the diamond-like carbon (DLC) film WAS OF Rz 5.82 μm and Rz 5.84 μm, respectively, that is, higher than Rz 4.0 μm. FIG. 1C shows that after the tests the surface of the cylinder at its upper dead center (UDC/PMS) was completely worn, without the presence of the hard carbon film. Additionally, the region adjacent below the upper dead center exhibited film delamination. The result of such tests showed clearly that roughness of cylinders with hard carbon films higher than Rz 4.0 μm undergo premature failure of the film, and so one should not exceed this roughness value.
It should be noted that the upper dead center (UDC) (see FIGS. 7A and 8) is the highest position, defined on the cylinder or liner, of the stroke of the piston rings (see FIGS. 7A, 7B and 8) with a diameter D. At the UDC position, an inversion of relative movement takes place on the first groove wing. This is the most stressed liner region, as far as wear is concerned, and a depression that can be observed as polishing of this region due to wear is generally formed, mainly in the case of engines that undergo greater pressures like those operating on Diesel, thus enhancing the formation of callus. Of course, in opposition to the upper dead center (UDC), there is the lower dead center (LDC), which, in the particular case of FIG. 7A and 7B, describes a compression rate in the piston-cylinder system. Thus, there is still no cylinder provided with a hard carbon film capable of finding a balance condition between the film roughness, friction coefficient and film wear and that overcomes the problems existing in the prior-art technologies, guaranteeing long durability in any internal combustion engine at a reduced cost.